The present invention relates to wafer testing and in particular to controlling temperature in a wafer during testing.
In the semiconductor integrated circuit industry, the cost of individual integrated circuit chip die is continuing to decrease in comparison to IC package costs. Consequently, it is becoming more important to perform many IC process steps while the die are still in the wafer, rather than after the relatively expensive packaging steps have been performed.
Typically, in IC processing, semiconductor wafers are subjected to a series of test and evaluation steps. For each step, the wafer is held in a stationary position at a process station where the process is performed. For example, circuit probe testing is increasingly performed over a wide temperature range to temperature screen the ICs before assembly into a package. The wafer is typically held stationary relative to a vacuum support surface of a prober machine which electrically tests the circuits on the wafer. The prober includes a group of electrical probes in a probe head which is positioned such that the probes are brought into contact with predetermined contact points on individual circuits formed in the wafer. The probes, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits"" responses to the excitations.
In a typical prober system, the wafer is mounted on the top surface of a wafer chuck, which is held at its bottom surface to a support structure of the prober. A vacuum system is typically connected to the chuck. A series of channels or void regions in communication with the top surface of the chuck conduct the vacuum to the wafer to hold it in place on the top surface of the chuck. The prober support structure is then used to locate the wafer under the probes as required to perform the electrical testing on the wafer circuits.
The chuck can also include a temperature control system which raises and lowers the temperature of the chuck surface to maintain the chuck surface at a desired set point temperature. Temperature control systems for the chuck can take several forms. For example, the chuck can be outfitted with electrical heaters which are controlled to provide heat to the chuck and raise the temperature of the wafer. The chuck can also include a heat sink for removing heat from the chuck. The temperature control system can also include a means for circulating a fluid through the chuck to cool and/or heat the chuck.
The set point temperature of the chuck is selected based on a desired testing temperature for the wafer. The chuck typically includes a temperature sensor mounted within the chuck in close proximity to its top surface and, therefore, the wafer mounted thereon. Since the temperature sensor is located close to the wafer, it is assumed that the temperature at the top surface of the chuck is the same as the temperature of the wafer circuit being tested and that any difference in temperature between the two is negligible.
In many modern circuit testing settings, this assumption can lead to inaccurate testing results. For example, in many circuits, power dissipation is relatively high. This results in substantial localized heating of the circuit or adjacent circuits when the circuit testing excitation signal is applied by a tester operating with the prober. The result is that the circuit is actually tested at a much higher temperature than the temperature set by the system, and the accuracy of the test is reduced.
Therefore, it would be beneficial to have a temperature control system for controlling the temperature of a wafer under test in which temperature sensing is realized at the actual circuit or wafer being tested.
The present invention is directed to an apparatus and method for controlling the temperature of at least a portion of an integrated circuit wafer which overcomes the drawbacks of the prior art. The apparatus of the invention includes a temperature sensing device integrally formed in the integrated circuit wafer. A sensing circuit receives from the temperature sensing device a signal indicative of temperature of the integrated circuit wafer. A temperature control medium is in thermal communication with the wafer. The temperature of the temperature control medium is controlled by a temperature control system. The temperature control system uses the signal indicative of the temperature of the wafer to control the temperature of the temperature control medium such that the temperature of the wafer is controlled.
In one embodiment, the temperature control medium is a temperature-controlled workpiece chuck on which the wafer is supported. In another embodiment, the temperature control medium is a stream of a temperature-controlled gas such as air which is directed onto the wafer.
The temperature sensing device formed in the wafer can be a temperature-sensitive diode. An excitation current can be driven through the diode by the prober tester, and the resulting voltage drop across the diode can be sensed. In such a diode, the voltage drop across the diode changes with temperature. Therefore, the sensed voltage drop can be used to generate the signal indicative of temperature of the wafer.
Hence, in one embodiment, the sensing circuit can be part of a testing circuit in a wafer prober machine used to test integrated circuits formed in the wafer. The testing circuit can be coupled to the temperature control system. The signal indicative of temperature of the wafer is used to generate a control signal which is applied to the temperature control system and is used by the temperature control system to control the temperature of the temperature control medium, e.g., the chuck or the air stream.
The temperature control approach of the invention can be applied to testing multiple individual integrated circuit die while they are still attached within a wafer. Accordingly, each individual circuit is formed with its own integral temperature sensing device, i.e., diode. As each circuit is tested by the prober, its respective temperature signal is sensed at its respective integral diode. The temperature for the particular circuit under test can then be adjusted based on its locally sensed temperature, thus assuring highly accurate temperature data for each test.
In one embodiment, the temperature control process used in the invention directly controls the temperature of the temperature control medium, i.e., chuck or air stream, to control the temperature of the wafer. That is, in the chuck system, the temperature of the chuck is controlled to reach and stabilize at a desired chuck temperature set point. If the chuck is at its desired set point and the wafer is not at its desired temperature, then a new set point is selected for the chuck. The set point for the chuck continues to be adjusted to maintain the temperature of the wafer at its desired temperature. Likewise, in the air stream system, the temperature of the air stream is sensed, and the sensed temperature is used to control the temperature of the air stream to maintain it at a desired set point. If the air stream temperature is at its set point and the wafer is not, then a new air stream set point is selected. This continues until the desired temperature is reached. Hence, in either system, the set point for the temperature control medium, i.e., chuck or air stream, is in general different than the set point for the wafer.
This type of control can be regarded as a xe2x80x9cdual-loopxe2x80x9d control in which two types of feedback are used to achieve the ultimate desired output. Such dual-loop temperature control, which can be used in accordance with one embodiment of the invention, is described in, for example, U.S. Pat. No. 4,734,872, entitled, xe2x80x9cTemperature Control for Device Under Test,xe2x80x9d issued Mar. 29, 1988, assigned to Temptronic Corporation, and incorporated herein in its entirety by reference.
The temperature control approach used in accordance with the invention is used to precisely control the temperature of the wafer circuit being tested, with temperature sensing being realized at the circuit under test, not at a relatively remote location. As a result, the temperature control tests performed in connection with the temperature control of the invention are more accurate than tests performed using prior approaches to temperature control.